it uses the headphone jack as the ADC. the app is called huskerscope.
i made this little board to allow me to have an input
I imagine the ADC has a high-pass filter. The low sample rate (usually 48kHz) could also be an issue
You can also use your phone as a 2-channel function generator using https://play.google.com/store/apps/details?id=com.keuwl.functiongenerator. Maybe you could use both at the same time to measure a circuit's frequency response